How to Make It
Heat oven to 350° F. Rub the oil on the bottom of a rimmed baking sheet.
Using a sharp paring knife, cut ½-inch-deep slits all over the lamb, about every inch or two. (Don’t skimp; you should make about 50 cuts.) Stuff the garlic into the slits. Season with 2 teaspoons salt and 1½ teaspoons pepper. Let sit at room temperature for 45 minutes (to deepen the flavors and to allow the meat to come to room temperature so that it roasts evenly).
Place the lemon slices, slightly overlapping, on the prepared sheet; top with 2 of the rosemary sprigs. Top with the lamb and the remaining 3 rosemary sprigs. (Make sure that all the lemon slices are tucked under the lamb to keep them from burning.)
Roast the lamb, basting with the pan juices every 15 to 20 minutes (beginning halfway through the cooking time), until an instant-read thermometer inserted in the lamb (avoiding the bone) registers 145° F, 1½ hours to 1 hour, 45 minutes.
Loosely tent the lamb with foil and let rest for 20 minutes before slicing perpendicularly to the bone. Serve warm or at room temperature.